The activation/inactivation of CDK by cyclins drives the transition between the different phases of the cell cycle. CDK activity is also regulated by CDK inhibitors known as KRPs (Kip-related proteins, wherein “Kip” is short for “kinase inhibitor proteins”). Other CDK inhibitors from plants and animal systems are known as CKI, ICK, Cip, and Ink (De Veylder, 2001, Plant Cell, 13:1653-1667) and KIS (Jasinski et al., J. Cell. Sci., 115:973-982 (2002)). CDK inhibitors have been identified in plants, including Arabidopsis and tobacco. The CDK inhibitors of plants have approximately 35 amino acids at the carboxy terminus homologous to the amino-terminal cyclin/CDK-binding domain of animal CDK inhibitors of the p21Cip1/p27Kip1/p57Kip2 types. Outside of the carboxy-terminal region, the plant CDK inhibitors identified thus far are structurally different.